In an image forming apparatus, upon discharge of paper on which an image formation has been completed to the outside of the apparatus, especially in high-speed image formation, a phenomenon such as floatage of the end of discharged paper or downward curl of both side edges of the paper happens in some cases. Accordingly, in some cases, paper-discharge failure that the discharged paper does not arrive at a predetermined position on a paper discharge tray, or paper stacking failure that the discharged papers are not stacked in an appropriate state occurs.
To solve this problem, the inventors of the present invention invented a paper discharge roller as shown in FIGS. 5 and 6. The paper discharge roller shown in FIGS. 5 and 6 is not known upon submission of priority Japanese patent application the present application claims the benefit thereof. This paper discharge roller 100 is set in front of a paper discharge tray of an image forming apparatus, and it has an upper roller 101 as a driving roller and a lower roller 102 as a driven roller.
The lower roller 102 of the paper discharge roller 100 has a nip roller 103 in a cylindrical shape having a constant outer diameter, and forming rollers 104, 104, in a tapered shape where the outer diameter is increased outward, integrally provided at both outer end surfaces of the nip roller 103. In this manner, the lower roller 102 is a part where the nip roller 103 at the center and the forming rollers 104, 104 at the both outer sides are integrated, and is provided rotatably with respect to a driven shaft 105. The upper roller 101 as a driving roller, having approximately the same shape as that of the nip roller 103, is arranged above the nip roller 103 at the center of the lower roller 102.
Upon paper discharge, by holding and conveying paper between the upper roller 101 as a driving roller and the lower roller 102 as a driven roller, both side edges of the paper parallel to a conveyance direction are deformed to be floated upward along the forming roller 104 of the lower roller 102 having an outer diameter larger than that of the nip roller 103 and the upper roller 101, and the paper as a whole is formed in a wavy shape. With this forming, the paper has a strong body, and the discharged paper accurately arrives at a predetermined position on a paper discharge tray, or is stacked in an appropriate status on the paper discharge tray. That is, it has been considered that it is possible to prevent paper discharge failure and discharge paper stacking failure by improving floatage by forming the paper in a predetermined shape by holding and conveying the paper on which an image has been formed between the both rollers 101 and 102 so as to discharge the paper having a strong body.
However, when the inventor continued further research and development regarding the paper discharge roller 100 shown in FIGS. 5 and 6 proposed by the inventors, the inventors further found that the paper discharge roller had the following problems. That is, since the lower roller 102 has a shape where the outer diameter of the both ends is larger than that of the central part for the purpose of paper forming, when it is rotated as a part at the same number of revolutions upon paper holding and conveyance, the linear velocity of the forming roller 104 having a relatively large outer diameter is higher than that of the nip roller 103 having a relatively small outer diameter. Accordingly, since the forming roller 104 is brought in friction-contact with paper surface, when an image is formed in the corresponding part of the paper, image forming material such as ink is transferred from the paper to the forming roller 104 and accumulated there. Thereafter, when the paper on which the image formation has been completed is passed through the paper discharge roller 100, the ink or the like is retransferred to the paper and the paper is contaminated.
The inventions described in the following Patent Literatures 1 and 2, unlike the paper discharge roller 100 having the forming roller 104 as described above, relate to prevention of contamination of a general paper conveyance roller. The Patent Literature 1 discloses that an outer peripheral layer formed of microscopic particles or an outer peripheral layer formed of an ink repellent individual material is formed on a timing roller which may be contaminated with ink, so as to prevent attachment of ink and contamination of paper. Further, the Patent Literature 2 discloses that a large number of projections are uniformly formed on peripheral surfaces of driving roller and driven roller. When holding and conveying a print-completed sheet, the rollers and the sheet are almost in point-contact, so as to solve the problem of contamination of the roller.    Patent Literature 1: Japanese Published Unexamined Patent Application No. 2005-66900    Patent Literature 2: Japanese Published Unexamined Patent Application No. 2007-137561
According to the inventions described in the above-described Patent Literatures 1 and 2, the peripheral surface of the roller is processed so as to prevent attachment of ink. However, the effect of the process is gradually reduced in the course of use. Further, in the paper discharge roller proposed by the inventors of the present application, where the outer diameter at both ends is large to shape paper to have a strong body, the problem of likely attachment of ink to especially the large outer diameter forming part is not directly solved. Even when the process is applied to the paper discharge roller proposed by the inventors, which attains excellent paper discharge, it cannot be considered possible to attain some effect.